1. Field of the Invention
The present invention relates to an image reading device that reads an original document image via a transparent member by moving the original document relative to the transparent member, and more particularly to an improvement of the technology to detect foreign particles on the transparent member.
2. Description of the Related Art
Image reading devices used in copying machines, for example, can be roughly divided, based on the reading method used, into (i) those using the so-called sheet-through method in which an image reading unit including a CCD sensor or the like is fixed at the original document reading area of the platen glass (i.e., the transparent member) and the image of the original document is read by moving the original document in the secondary scanning direction at a prescribed speed using an automatic original document feeder, and (ii) those using the so-called scanner movement method in which the original document image is read by moving the image reading unit while the original document is fixed in place.
The scanner movement method requires space and a mechanism for the movement of the image reading unit because the image reading unit is moved over the full length of the original document in the secondary scanning direction, while the sheet-through method offers the advantage that the space, etc. required for movement of the image reading unit is not required, enabling the device to be made smaller and less expensive in comparison with the scanner movement method.
However, in the case of the sheet-through method, if a foreign particle is present at the original document reading area on the platen glass due to introduction of dust, the foreign particle is read at all times during original document reading, even if it comprises only a small dot, which leads to the same result as when the original document is soiled from one end to the other in a linear fashion, resulting in continuous linear noise running in the secondary scanning direction in the image reproduced from the image data obtained by reading the original document.
Accordingly, Japanese Patent Laid-Open No. 2002-271631 discloses an image reading device based on the sheet-through method that reads the color image for each pixel, in which a reading CCD sensor to read the color components of R (red), G (green) and B (blue) and a foreign particle detection CCD sensor to detect foreign particles on the platen glass are disposed along the secondary scanning direction with a prescribed distance therebetween, the image information (i.e., the density value data) output from the reading CCD sensor and the image information output from the foreign particle detection CCD sensor are compared for each pixel, foreign particles on the platen glass are detected from the comparison results, and correction is made accordingly.
More specifically, focusing on one pixel of the original document image, if there are no foreign particles on the platen glass, the image information output regarding this pixel by the respective CCD sensors should be essentially identical. However, if a foreign particle resides on a part of the platen glass that constitutes a part of the light path to the reading CCD sensor, the image information from the reading CCD sensor that is affected by the foreign particle and the image information from the foreign particle detection CCD sensor that is not affected by the foreign particle are conspicuously different from each other, and where there is such a difference, the foreign particle on the glass is detected. When a foreign particle is detected, correction is made to the image information for the pixel that is affected by the foreign particle such that the foreign particle does not appear in the image as noise.
However, according to the image reading device disclosed in Japanese Patent Laid-Open No. 2002-271631, a CCD sensor must be included in the device for the detection of foreign particles, in addition to the CCD sensor for reading. Furthermore, if dust, for example, adheres to parts of the platen glass within the light paths of both CCD sensors, no conspicuous difference arises between the image information output by the reading CCD sensor and the image output by the foreign particle detection CCD sensor, making detection of foreign particles impossible.